Subsurface absorption of anthropogenic warming of the land surface: The case of the world's largest brickworks (Stewartby, Bedfordshire, UK)

Stewartby works, for a time the world's largest brickworks, began operation around the start of the twentieth century and closed in 2008. Subsurface temperature measurements are available in its vicinity, obtained as part of monitoring of an adjacent landfill in one of the former quarries for the Oxford Clay, which was the raw material for brick manufacture. A striking subsurface temperature anomaly, an increment of ~ 12 °C, was first measured in 2004, and has subsequently decayed over time. The anomaly is centred beneath one of the former brick kilns, which operated between 1935 and 1991. To investigate processes of heat absorption by the shallow subsurface, this anomaly has been modelled as a consequence of conductive heat flow into the ground due to the operation of the ~ 3000 m<sup>2</sup> kiln. This modelling indicates that a very large amount of heat energy was transported into the subsurface; we estimate the typical downward surface heat flow during operation of the kiln as ~ 1 W m <sup>−2</sup> and the energy stored in the subsurface beneath it at its time of shutdown as ~ 6 TJ, or ~ 0.03% of that released by the fuel for heating the kiln, such that the total heat energy stored beneath this multi-kiln site peaked at ~ 200 TJ. The proportion of heat energy transported into the subsurface was relatively low due to the nature of the Oxford Clay, which has a low thermal conductivity (~ 0.8 W m <sup>−1</sup> °C <sup>−1</sup>) and diffusivity (~ 0.3 mm<sup>2</sup> s <sup>−1</sup>); in a more conductive lithology it might well have been three times greater. After kiln shutdown this subsurface thermal anomaly began to dissipate by upward heat conduction and release of heat into the atmosphere; at present about half of the peak energy stored remains, decreasing at ~ 1% per year, the maximum temperature anomaly being currently ~ 7 °C at a depth of ~ 30 m and the typical upward heat flow during this span of time having exceeded the regional ~ 40 mW m<sup>− 2</sup> background by roughly an order of magnitude. We believe this to be the first documented case whereby a subsurface thermal anomaly associated with operation of industrial plant has been related in detail to the history of site operations. This case study thus bears upon the controversial topic of the development of subsurface heat islands in general, and the associated perturbation of the thermal state of the subsurface as a result of anthropogenic warming of the atmosphere. It has previously been suggested that the worldwide heat gain in the subsurface over recent decades has exceeded that in the atmosphere by a factor of three. We show that this result is subject to some uncertainty, for example because it does not factor in lateral variations in thermal properties. Nonetheless, our case study demonstrates dissipation of a subsurface thermal anomaly by heat transport into the atmosphere. This indicates that warming of the atmosphere will be sustained in the future by dissipation of the large amount of energy stored in pre-existing subsurface thermal anomalies on a global scale, an issue of major societal implications that demands more detailed investigation